The IP Multimedia Subsystem (IMS) is a standardized networking architecture for the 3rd Generation Partnership Project (3GPP) network and future generation mobile networks. The IMS provides users of mobile communication devices with mobile and fixed multimedia services, such as application services. The IMS runs over the standard Internet Protocol (IP), using a Voice-over-IP (VoIP) protocol based on a 3GPP standardized implementation of Session Initiation Protocol (SIP). SIP is a protocol developed for initiating, modifying and terminating an interactive user session that involves multimedia elements, such as video, voice, instant messaging, gaming, and virtual reality.
The IMS architecture enables a user of a mobile communication device to connect to an IMS network, regardless of what access network the user is using, as long as the access network supports IP. Accordingly, an IMS network can be accessed via any network with packet-switching functionality, such as General Packet Radio Service (GPRS), Universal Mobile Telecommunications System (UMTS), Code Division Multiple Access (CDMA), Wireless Local Area Network (WLAN), WiMax, Digital Subscriber Line (DSL), cable, etc. Also, the IMS network may be accessed through circuit-switched telephone systems, like the Public Switched Telephone Network (PSTN) and the Global System for Mobile Communications (GSM), that are supported through appropriate gateways. Direct IMS terminals, such as mobile phone personal digital assistants (PDAs), computers, etc., can register directly into an IMS network, as long as they support SIP agents.
An important aspect of the IMS is enhanced user mobility, which allows operators and service providers to use different underlying network architectures, such that the mobile network provides terminal mobility (roaming), but user mobility is provided by the IMS and the SIP.
In a conventional 3GPP network, a user and his or her mobile communication device are identified and authenticated using identifiers including an International Mobile Subscriber Identity (IMSI), which is a unique user identity of a Universal Subscriber Identity Module (USIM) of the mobile terminal, and a Mobile Subscriber ISDN Number (MSIISDN), which is the actual telephone number of the user. The USIM is an application that runs on the universal integrated circuit card (UICC) of the mobile communication device. The UICC is commonly referred to as the Subscriber Identity Module (SIM) card.
The authentication model used in the current IMS architecture is patterned after the conventional 3GPP network. In the current IMS architecture, a user and his or her mobile communication device are identified and authenticated using identities including an Internet Protocol Multimedia Private Identity (IMPI) and an Internet Protocol Multimedia Public Identity (IMPU). Instead of phone numbers, these identifiers typically include Uniform Resource Identifiers (URIs). The IMPI is the private identity of the Internet Protocol Multimedia Service Identity Module (ISIM), which is an application than runs on the UICC of the mobile communication device. The IMPI is unique to the terminal device, but a user may have multiple IMPUs per IMPI.
The user database of the IMS, also referred to as the Home Subscriber Server (HSS) contains at least the IMPU, the IMPI, the IMSI, and the MSIISDN. During registration of a mobile communication device to the IMS, the IMPI and the IMPU are sent with registration requests to the IMS. The IMPI is authenticated to confirm the identity of the user using the IMPU to gain access to the IMS.
As noted above, the IMSI has traditionally been stored securely in the UICC or SIM card of a mobile terminal. By default, the IMPI has also been stored in the UICC or SIM card.